The 44-year-old Scot had seven birdies to come from four shots behind and finish with an 11-under 269 and win his first title since the Hong Kong Open in December 2005.
Niclas Fasth (67) of Sweden finished second, one shot back. Anthony Wall (65), Pelle Edberg (66), Peter Hanson (68) and Gregory Havret (68) were tied for third at 271.
Montgomerie sank 6-foot par putts at the 17th and 18th holes, and then had to wait nearly two hours to confirm his victory after an 80-minute thunderstorm delay. An earlier one had lasted 45 minutes.
'I'm thrilled with this success,' Montgomerie said. 'I must admit the last two holes I got fortunate. I made two good 3s.
'This was a very, very important win. After such a dry spell you wonder if it's ever going to happen again. You do have self doubts. You wonder if it will happen and I'm so glad it has.'
Montgomerie birdied three holes on the front nine and four on the back to win his 31st European Tour tournament -- one more than Nick Faldo.
Faldo is 'someone who I have respected for many years and to have one more victory on the European Tour than him means an awful lot to me,' Montgomerie said.
Fasth, who won in Germany two weeks ago after finishing fourth in the U.S. Open, missed 8- and 14-foot putts for birdie at the last two holes. He had battled with the flu during the first three rounds.
'I played absolutely fantastic today and felt I gave it a good strong go,' Fasth said. 'I left some 8- or 10-footers on the edge and it didn't really happen today like it needs to for you to win a tournament.
'I just tried to pull through and I nearly got it done.'